1. Field of the Invention
The present invention relates to an image processing technique enabling a constituent of a subject in a captured image for medial use to be discriminated.
2. Description of the Background Art
In a radiographic image used for medical diagnosis, an image density proportional to a total amount of energy spectrum detected by a detector in a radiographic apparatus (hereinafter, referred to as “gray image”) is formed. In the gray image, a pixel density IkV of an image obtained by collecting when a tube voltage of an X-ray tube of the radiographic apparatus is kV can be expressed as the following equation (1) using energy of X-ray (hereinafter, referred to as X-ray energy) E.
                              I          kV                =                  ϰ          ⁢                                    ∫              0              kV                        ⁢                                                            EP                  kV                                ⁡                                  (                  E                  )                                            ⁢              exp              ⁢                              {                                                      -                                          μ                      ⁡                                              (                        E                        )                                                                              ⁢                  L                                }                            ⁢                              ⅆ                E                                                                        (        1        )            
PkV(E) denotes the number of photons of X-ray (hereinafter, referred to as X-ray photon number) per pixel detected by the detector at X-ray energy E, μ(E) denotes a linear attenuation coefficient [cm−1] of the subject through which the X-ray passed, and L denotes a thickness [cm] of the subject. A constant κ denotes a system gain for converting a total amount of the X-ray energy integrated by the detector of the radiographic apparatus to pixel density.
In a conventional technique, a two-dimensional distribution of Ikv is provided for diagnosis. Based on light and shade information of an image which changes according to the linear attenuation coefficient of a substance constituting the subject, a position of a bone or an organ, a presence or absence of a lesion, and the like is diagnosed.
However, since the attenuation of an X-ray is determined by a product of the linear attenuation coefficient of the substance and the thickness of the substance as expressed by the equation (1), if the products are the same value, the densities of images are the same. In other words, when it is assumed that there are two kinds of substances A and B, if the product of the linear attenuation coefficient and the thickness of the substance A and the product of the linear attenuation coefficient and the thickness of the substance B are the same, X-rays passing through the substances A and B attenuate to a similar extent, so that the substances A and B cannot be discriminated from each other. Therefore, when the substance A is a bone and the substance B is a tumor, even if the tumor develops in a portion of the bone with a thin thickness, there is a case such that an image density of the tumor is almost equal to that of a portion of the bone around the tumor. For example, in a case such that a tumor develops fortuitously in a portion having a low bone density or a thin thickness of the bone, the image density hardly changes in a gray image, so that it is impossible to find the tumor in the gray image. Moreover, in a case of a chest X-ray diagnosis, in the diagnosis of a gray image, the density of a display image of a tumor overlapped on a diaphragm or a mediastinum is low, and the tumor cannot be represented with sufficiently high contrast. Consequently, a tumor is not easily found and is sometimes overlooked.
To address the drawback, as a first conventional technique, a technique of eliminating a shadow of a bone by a so-called energy subtraction method and forming an image of only a soft tissue is widely known. The energy subtraction method is a method of collecting two kinds of radiographic images of the same subject while changing quality of an X-ray, giving different weighting factors to images so that the attenuation of the X-rays caused by a bone become the same, and performing subtraction (refer to, for example, Japanese Patent Application Laid-Open No. 05-181954).
However, in the energy subtraction method, a beam hardening phenomenon occurs such that the linear attenuation coefficient of a bone changes apparently according to the thickness or the like. It is therefore impossible to completely eliminate a bone having various thicknesses. In addition, since a characteristic value itself of an image obtained by the energy subtraction method is meaningless, light and shade information of images is various in image capturing operations. Accordingly, only a relative evaluation in a single image can be performed.
As a second conventional technique for improving the energy subtraction method, a radiation diagnostic apparatus is proposed, which can recognize that composition substances of a subject are different from each other even if radiation absorption amounts thereof are the same by obtaining a radiation absorption coefficient of a subject when the thickness of the subject is equal to that of a reference substance, calculating a difference between the obtained radiation absorption coefficient and a radiation absorption coefficient of a reference material, and obtaining an image of the difference (refer to, for example, Japanese Patent Application Laid-Open No. 05-161632).
As a third conventional technique, a radiation diagnostic apparatus is proposed, which can recognize that composition substances of a subject are different from each other even if X-ray transmission amounts thereof are the same by obtaining a change in an X-ray transmission amount relative to a change in the energy of an X-ray in a plurality of regions pixel by pixel (refer to, for example, Japanese Patent Application Laid-Open No. 05-161631).
Further, as a fourth conventional technique, a radiation diagnostic apparatus of obtaining an average absorption coefficient of a subject from images captured with X-rays of a plurality of energies and calculating a substance existence amount pixel by pixel is proposed (refer to, for example, Japanese Patent Application Laid-Open No. 05-161633).
In the second to fourth conventional techniques, the drawback that an error easily occurs in diagnosis due to various thicknesses of a bone like in a simple energy subtraction method is improved.
However, in the second conventional technique, process is performed based on image data obtained by the energy subtraction method. An obtained value itself is meaningless, and the light and shade information of an image varies according to the image processing method. Therefore, only a relative evaluation can be made.
In the third conventional technique, only changes in a region subjected to image capture are compared. Further, in the fourth conventional technique, an existence amount of a substance per region based on the X-ray average absorption coefficient is a display target. Similar to the first conventional technique, a value itself is meaningless, obtained data varies according to the image processing methods, and only a relative evaluation can be made.